Sheet-fed recto-verso printing presses are known in the art, in particular for performing simultaneous recto-verso printing of sheets or webs.
Swiss patent CH 502 897 discloses a multi-colour recto-verso printing press for performing simultaneous recto-verso offset printing. The press comprises two blanket cylinders contacting one another to form a printing nip where the paper is printed, each blanket cylinder carrying inked patterns to be applied on to the paper. The inked patterns are formed on the surface of the blanket cylinders by means of two groups of inking devices and plate cylinders. Sheets to be printed are fed to the printing location, between the two blanket cylinders, and are transferred, once printed, to a sheet delivery system, typically a so-called chain gripper systems comprising a plurality of spaced-apart gripper bars comprising a series of grippers for holding a leading edge of the sheets. Another example of a similar printing presses can be found in European patent application EP 0 949 069 A1.
For the purpose of ensuring a sufficient level of printing quality, it is often desired to carry out inspection of the freshly printed sheets on the printing press itself. One typically speaks in this case of in-line inspection, in contrast to off-line inspection where sheets are inspected separately from the printing press or presses where they were printed. In-line inspection system are for instance described in European patent applications EP 0 527 453 and EP 0 576 824. EP 0 576 824 in particular describes an in-line inspection system for carrying out inspection on a recto-verso printing press. In this example, inspection is carried out in the sheet transfer path between the printing station and a sheet-delivery station of the printing press.
Carrying out in-line inspection on recto-verso printing presses is rather complicated because both sides of the printed sheets are printed with fresh ink. Indeed, for inspection to be carried out, one must ensure that, during inspection, the printed sheet to be inspected or at least part of it is appropriately positioned with respect to a reference location. With single-sided printing press, the side of the sheet which has not been printed with fresh ink can be drawn against a reference surface, typically by means of suctions means, inspection of the other side of the sheet being carried out while the sheet is held against the reference surface. Such solution is described in the above-mentioned European patent application EP 0 527 453. With double-sided printing presses, such a solution can only be envisaged if the side of the sheet that is drawn against the reference surface has previously been dried. Otherwise, the contact of the freshly printed side of the sheet with the reference surface would cause smearing and therefore irremediably degrade the printing quality of that side of the sheet.
The solutions proposed so far for carrying out in-line inspection on recto-verso printing presses typically consist in locating the inspection system in the sheet-delivery path of the printing press, as proposed in European patent application EP 0 576 824. Further examples of such a principle can be found in European patent applications EP 1 142 712, EP 1 167 034 and EP 1 323 529. A problem with such solutions however resides in the increased complexity of the printing press. This is particularly the case of the solutions described in EP 1 142 712, EP 1 167 034 and EP 1 323 529 which require two separate sheet transport systems, namely a first one to transfer the sheets from the printing station to the sheet inspection system and a second one to transfer the sheets from the inspection system to the sheet-delivery station.
Still another problem of the prior art solutions resides in the increased length of the sheet delivery path caused by the presence of the sheet inspection system itself, which thereby increases the footprint of the printing press as a whole.
There is therefore a need for a less complicated and more compact solution.